1. Field of the Invention
The invention relates to a dual-band antenna, and more particularly, to a dual-band antenna with three resonators.
2. Description of the Prior Art
Radiotelephones generally refer to communications terminals that provide a wireless communications link to one or more other communications terminals. Radiotelephones are utilized in variety of different applications, including cellular phones, satellite communications systems, and so forth. Radiotelephones typically have an antenna for transmitting and/or receiving wireless communications signals.
Radiotelephones and other wireless communications device are undergoing constant miniaturization. Thus, there is an increased demand in small antennas that can be used as internally mounted antennas for radiotelephones. In addition, it is becoming desirable for radiotelephones to be able to operate within multiple frequency bands in order to utilize more than one communications system. For example, GSM (Global System for Mobile communication) is a digital mobile telephone system that typically operates at a low frequency band, such as between 880 MHz and 960 MHz. DCS (Digital Communications system) is a digital mobile telephone system that typically operates at a high frequency band, such as between 1710 MHz and 1880 MHz. Since there are two different frequency bands, radiotelephone service subscribers who travel over service areas employing different frequency bands may need two separate antennas unless a dual-band antenna is used. Additionally, as the amount of data being sent through wireless communications signals increases, the bandwidth of the frequency band at which the antenna operates is required to increase as well.
Please refer to FIG. 1. FIG. 1 is a perspective view of a prior art antenna 10 disclosed in U.S. Pat. No. 5,926,139. The prior art antenna 10 comprises a conductive ground plate 14, a conductive first plate 12 set above the ground plate 14, a conductive connector 18 having two opposite ends connected to the ground plate 14 and the first plate 12, and a signal feeder 19 having two terminals. One terminal of the signal feeder 19 is a grounded terminal electrically connected to the ground plate 14, and the other terminal is a signal terminal 16 electrically connected to the first plate 12. Data signals, which are transmitted from the antenna 10 or received by the antenna 10 are fed through the signal feeder 19. The connector 18 is a short pin for connecting the first plate 12 and the ground plate 14. For operating within two frequency bands, the first plate 12 of the prior art antenna 10 has two resonating regions 17A, 17B, each corresponding to one frequency band at which the antenna 10 operates. In addition, European Pat. No.EP0997974A1 discloses an antenna that is similar to the antenna 10 having the first plate 12 on which two resonating regions are disposed.
Please refer to FIG. 2. FIG. 2 is a correlation diagram between reflection and frequency of the prior art antenna 10. The horizontal axis represents the frequency, and the vertical axis represents the absolute value of reflection. The reflection of an antenna can be used to evaluate a bandwidth of a frequency band at which the antenna operates. Generally, a frequency range under reflection of xe2x88x9210 decibel (dB) is used to be the frequency band at which the antenna operates. As shown in FIG. 2, the two resonating regions 17A, 17B of the antenna 10 (shown in FIG. 1) respectively correspond to two frequency bands A1, A2 of the antenna 10 distributed around frequencies fa, fb so that the antenna 10 can operate within the two frequency bands A1, A2.
Since the prior art antenna 10 is planar, it is very suitable for embedding into portable wireless communications devices, such as a cellular phone, so as to rid the device of protruding antennas. However, the prior art antenna 10 has a disadvantage of narrow bandwidth, especially a narrow bandwidth at a higher frequency. For example, the specification of a frequency band distributed around 1800 MHz must have a bandwidth of 170 MHz. However, the antenna 10 with regular dimensions does not have enough bandwidth to meet the requirements of a digital mobile phone system that operates at a frequency band of 1800 MHz. Thus, in order to increase the bandwidth of the antenna 10, the dimensions of its corresponding resonating region are required to be enlarged. Unfortunately, enlarging the dimension of the resonating region will expand the physical area and the physical volume of the antenna 10. Expanding the size in this way will adversely affect the ability to miniaturize a cellular phone.
It is therefore a primary objective of the claimed invention to provide a dual-band antenna with three resonators to solve the above-mentioned problem.
According to the claimed invention, the antenna comprises a conductive ground plate, a conductive first plate, a conductive connector, and a signal feeder. The conductive first plate is set above the ground plate, and a fixed distance separates the first plate and the ground plate. The first plate comprises first, second, and third resonance regions with respective dimensions corresponding to wavelengths of first, second, and third frequencies at which the antenna operates. The first plate also comprises a connection region connected to the first, the second, and the third resonance regions. The conductive connector has two opposite ends respectively connected to the ground plate and the connection region. The signal feeder has two terminals respectively electrically connected to the ground plate and the first plate. The first, the second, and the third frequencies are different and respectively correspond to first, second, and third frequency bands of the antenna. The second frequency is close to the third frequency such that the second frequency band and the third frequency band are partially overlapped to cause the second frequency band and the third frequency band to merge.
It is an advantage of the claimed invention that the dual-band antenna with three resonators is capable of substantially broadening the bandwidth to overcome the prior art shortcomings.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.